CONFORMITY TO TYPE 249 



correspondence with the greater complexity of their structure it is 

 more complicated. 



"As the process of amphimixis occurs in them also, and the fission 

 of the highly differentiated multicellular individuals seems to be 

 only possible by a temporary return to the unicellular condition, 

 we find that the so-called 'sexual reproduction/ which is of 

 general occurrence among them, consists in all the primary con- 

 stituents (Anlagen) of the entire organism being collected together 

 in the nucleus matter of a single reproductive cell. 



"Two kinds of such cells, which are differently equipped and 

 mutually attract one another, then unite in the process of amphi- 

 mixis and constitute what we are accustomed to call the 'fertilized 

 egg cell,' which contains the combined hereditary substance of 

 two individuals. According to our view, this hereditary substance 

 of the multicellular organisms consists of three orders of vital 

 units, the lowest of which is constituted by the biophors. In the 

 unicellular forms a more or less polymorphic mass of biophors 

 having a definite arrangement, constitutes the individual nuclear 

 rods or idants (chromosomes), several of these making up the 

 hereditary substance of the nucleus which controls the cells; and 

 similarly in these higher forms, groups of biophors, arranged in a 

 certain order, constituting the primary constituents of the in- 

 dividual cells of the body, and together form the second order of 

 vital units the determinants. The histological character of every 

 cell in a multicellular organism, including its rate and mode of 

 division, is controlled by such a determinant. The germ cell, 

 however, does not ^contain a special determinant for every cell 

 unless it is to remain independently variable. The germ cell of a 

 species must contain as many determinants as the organism has cells 

 or groups of cells which are independently variable from the germ 

 onwards, and these determinants must have a definite mutual 

 arrangement in the germ plasm, and must therefore constitute a 

 definitely limited aggregate, or higher vital unit, the ' id.' From 

 the facts of sexual reproduction and heredity we must conclude 

 that the germ plasm contains many ids, and not a single one only. 

 The formation of hybrids proves that the two parents together 

 transmit all their specific characters, so that in the process of 

 fertilization each contributes a hereditary substance which con- 

 tains the primary constituents of all parts of the organism that is, 

 all the determinants required for building up the new individual. 

 The hereditary substance becomes halved at the final stage of 

 development of the germ cells, and consequently all the deter- 

 minants must previously have been grouped into at least two ids. 

 But it is very probable that many more ids are usually present 

 and that in many cases their number far exceeds a hundred. It 

 cannot be stated with certainty which portions of the elements 

 of the germ plasm observable in the nucleus of the ovum correspond 

 to the ids, though it is probable that only parts of and not the 

 entire 'chromosomes' are to be regarded as such. Until this 

 point can be definitely decided, our further detailed deductions 

 will be based on the view that the nuclear rods (chromosomes) 

 are aggregates of ids, which we speak of as ' idants.' In a certain 

 sense these are also vital units for they grow and multiply by 



